Substation protection against direct strokes

Stations and substations contain costly equipment (transformers, breakers, busbars), so a direct lightning stroke must be intercepted before it reaches live parts and conducted to earth through a path of low impedance. The common methods are earthing screens, overhead shield wires and lightning masts, all bonded to the station earth mat.

1. Earthing screen

   ==+=====+=====+=====+==   earthed wire mesh
     |     |     |     |     above equipment
  [T/F]  [CB]  [Bus] [CT]
     |     |     |     |
  ======== earth mat ========

• A network (mesh) of copper or galvanised steel wires is erected over the whole substation on the steel structures.
• It is connected at many points to the low-resistance earth mat of the substation.
• A direct stroke hits the screen instead of the equipment and the current flows to earth.
• Advantages: simple and cheap; covers the whole yard.
• Limitations: it does not protect against travelling waves entering through incoming lines; if the earth resistance is high, the potential of the screen rises and can cause back flashover to equipment.

2. Overhead ground wires extended over the station

• The shield wires of incoming lines are carried over the substation and earthed on the gantries.
• Equipment lies within the protective zone (shielding angle of about 30°–45° under the wire).

3. Lightning masts / spikes

• Tall earthed steel masts or spikes on gantries (Franklin rods) placed so that all equipment is within the cone or rolling-sphere protection zone.
• Each mast has its own earth connection bonded to the earth mat.

Design points

1. Earth mat resistance should be low (typically below 1 Ω for large substations).
2. Adequate clearance between shield wires/masts and live parts to avoid back flashover.
3. Down conductors short and straight to reduce inductance.
4. Protective zone checked by the protective-angle or rolling-sphere method.

Protection against direct strokes is always used with lightning arresters at line entrances and transformer terminals, which protect against surges travelling in along the lines.